5AHP7A Vintage SSTV The Deep image

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Re: 5AHP7A Vintage SSTV The Deep image

Postby Steve Anderson » Tue Jul 05, 2016 11:44 am

Indeed, there are many ways to consider how the 'S-curve' demodulator does its job, you could spend a long time analysing it and possibly end up being none the wiser. That is why I left the two filters as they were. All I did was improve(?) the full-wave rectification and change the rather strange original output filter.

The 'new' demod method. I doubt that it's new, I'm sure it's been done before though I've not seen any mention of it anywhere so far.

I've made a start on the hardware and possibly, with no interruptions, I might have something to put forward tomorrow.

Steve A.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Wed Jul 06, 2016 1:53 pm

Steve Anderson wrote:Indeed, there are many ways to consider how the 'S-curve' demodulator does its job, you could spend a long time analysing it and possibly end up being none the wiser. That is why I left the two filters as they were. All I did was improve(?) the full-wave rectification and change the rather strange original output filter.

The 'new' demod method. I doubt that it's new, I'm sure it's been done before though I've not seen any mention of it anywhere so far.

I've made a start on the hardware and possibly, with no interruptions, I might have something to put forward tomorrow.

Steve A.


Yes also i am interested in your idea Steve interested to see how it turns out .

I was reading up on the old 4046 had me wondering if it could be used for SSTV demodulator ,could it be tuned to the bandwidth needed ?
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Steve Anderson » Wed Jul 06, 2016 3:14 pm

Hi Harry. The problem with using a PLL to demodulate the SSTV sub-carrier is that the modulation frequencies (DC to around 1kHz) and the sub-carrier frequencies (1200Hz-2300Hz, +/- side-bands) are so close together. This places an almost impossible task on the loop/low-pass filter. If the sub-carrier were (say) 10kHz-12kHz then it could be possible, though still not a cinch.

Although I had no interruptions yesterday I still ended up losing the best part of the day. My USB hub decided to quit on me and after a lot of mucking around I had to go out and buy a replacement. So not as much progress as I had hoped...a lot less!

I haven't forgotten the remaining circuits for your monitor that started this thread. I must make an effort to catch up!

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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Wed Jul 06, 2016 4:12 pm

Steve Anderson wrote:Hi Harry. The problem with using a PLL to demodulate the SSTV sub-carrier is that the modulation frequencies (DC to around 1kHz) and the sub-carrier frequencies (1200Hz-2300Hz, +/- side-bands) are so close together. This places an almost impossible task on the loop/low-pass filter. If the sub-carrier were (say) 10kHz-12kHz then it could be possible, though still not a cinch.


Oh oK I wasn't sure thats a bummer i saw some circuits using it in Fm radio receivers but as you say it was at the larger 10khz bandwidth .

Although I had no interruptions yesterday I still ended up losing the best part of the day. My USB hub decided to quit on me and after a lot of mucking around I had to go out and buy a replacement. So not as much progress as I had hoped...a lot less!


Both had a bad day by the sounds of it ! my clutch plate went in my ute yesterday all i can say is today is a much better day fingers crossed for better luck on your project today as well . :roll:

I haven't forgotten the remaining circuits for your monitor that started this thread. I must make an effort to catch up!

Steve A.


Thats ok Steve i was going to ask you a few questions onit ..... the High voltage dual power supply i am needing around 380 90 i think you mentioned .
Should be around that with the load resistors removed .
On the CRT supplies modulation is it again around a opto coupler as in your design here which would be close to something my tube would need adjusting the voltages to my P7 ,to me this looks very usable i know the heater supply is not needed as i am using 6,7 AC and negative supply will be half i would say ?
Also the sawtooth circuits would be a near copy of the 3bp1 ? apart from adjusting it to sstv line freq and frame rate ? just wondering as i was thinking this is what you had in mind curious i am :wink:
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Steve Anderson » Wed Jul 06, 2016 7:59 pm

A good bit of progress today, but nothing worth posting yet. Given time tomorrow I may have something, but I have to deal with some official business here tomorrow. On Friday I plan to get my glasses fixed as last week I on my bicycle and a car tried to occupy the same space at the same time - the car won. Oh well.

For the 5AHP7A there should be no need for an opto-isolator as apart from the nasty 7kV on the final anode, there's nothing on the CRT that semis can't handle directly. There's a good chance that one side of the heater and possibly the cathode could be earthed...or within a few dozen volts.

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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Wed Jul 06, 2016 9:59 pm

Harry, it is because of the magnetic deflection that the picture tube can be operated completely differently.

Using electrostatic deflection the deflection amplifiers should operate a a reasonable DC-voltage, say between +100 and +300 volt. That voltage is comparable to the final accelerator high voltage. So the cathode should be at a high negative voltage, and that is why the opto coupler is needed for the isolation.

Using magnetic deflection, the deflection coils are completely isolated from the HV circuit. Now the accellerator voltage may be +7 kV and all other electrodes of the picture tube can be at reasonable low voltages. To give an example:
- The cathode at +70 V,
- The grid at +30 V,
- The anode at +300V.

I did the video modulation in the cathode and the grid voltage is variable to control the brightness of black parts of the picture. The video modulator has a gain control (contrast) that keeps black (from the 1500 Hz input signal) at a fixed level. This is very handy, as you can control black and white with two separate potentiometers, which don't interact. From the FM-detector the video signal is DC coupled, there is no capacitor in the signal path.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Wed Jul 06, 2016 11:54 pm

Steve Anderson wrote:For the 5AHP7A there should be no need for an opto-isolator as apart from the nasty 7kV on the final anode, there's nothing on the CRT that semis can't handle directly. There's a good chance that one side of the heater and possibly the cathode could be earthed...or within a few dozen volts.

Steve A.
"Klaas Robers"]Harry, it is because of the magnetic deflection that the picture tube can be operated completely differently.


Ok Steve and Klaas,much easier on the modulation side for a magnetic type then ,i don't know of any large electrostatic television tubes were there any ? if so must of been the reason i suppose the voltages needed .....my little one was bad enough.

Using electrostatic deflection the deflection amplifiers should operate a a reasonable DC-voltage, say between +100 and +300 volt. That voltage is comparable to the final accelerator high voltage. So the cathode should be at a high negative voltage, and that is why the opto coupler is needed for the isolation.


Yes i can see now what you and Steve are saying that transistors at that voltage range can do the job.

Using magnetic deflection, the deflection coils are completely isolated from the HV circuit. Now the accellerator voltage may be +7 kV and all other electrodes of the picture tube can be at reasonable low voltages. To give an example:
- The cathode at +70 V,
- The grid at +30 V,
- The anode at +300V.


Its much easier to power these then ,i do like both types good to do hands on try and learn ,i have a few of these things laying around and when i come across them i will know more than i did yesterday working on them :wink:

I did the video modulation in the cathode and the grid voltage is variable to control the brightness of black parts of the picture. The video modulator has a gain control (contrast) that keeps black (from the 1500 Hz input signal) at a fixed level. This is very handy, as you can control black and white with two separate potentiometers, which don't interact. From the FM-detector the video signal is DC coupled, there is no capacitor in the signal path.


I know some use the grid and others the cathode to do the modulation as you mentioned is it really muchness on what you design it to do here or one way is better than the other ?.
Yes i noticed on the magazine designs the controls brightness such are easier for me to understand than on a electrostatic type .
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Thu Jul 07, 2016 5:57 pm

Harry, there are no larger TV picture tubes with electrostatic deflection. With electrostatic deflection the practical angles of deflection are small. Say 20 degrees. Then for a larger screen the tube becomes very long. Unpractical for a TV set. For magnetic deflection the angles are much larger, 70, 90, 110, 120 degrees, they increased during time.

On the other hand, a large bandwidth in deflection is very difficult to obtain with a coil. So for oscilloscopes, where flat bandwidth is needed from DC to 10, 30, 50, 100, 150 MHz, coils are not practical. Two deflection plates, which is a small capacitor, some pF, is doable. That is the reason oscilloscopes use long and narrow electrostatically deflected CRTs. They seldomly need beam modulation, except for the fly back suppression. But that is just a short pulse, which can be given to the high negative voltage of the grid or cathode via a relatively small capacitor.

So, that is the reason that there is a huge difference in CRTs for oscilloscopes and TVs.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Steve Anderson » Thu Jul 07, 2016 6:36 pm

Here's a pair of rather large ones...

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Re: 5AHP7A Vintage SSTV The Deep image

Postby Steve Anderson » Thu Jul 07, 2016 6:49 pm

I suspect that the CRTs in the photo above were lab development 'experiments' and not put into full-scale production. Especially when you consider the time-frame. Possibly a result of a good Friday lunchtime session down the pub!

Steve A.

Another result of some guys having too much time on their hands below...
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Thu Jul 07, 2016 10:02 pm

Hi Steve Klass i can see size does matter in CRTs seems to what they are used for most of the time !.
I like the ingenuity of the coke bottle lab CRT i looked for a link for it even if it was just for amusement ///i prefer pepsi my self :wink:
http://www.sm5cbw.se/tubes/htm/coketron.htm
I am not sure it worked the way from the description i was thinking
I have to admit i am Guilty as well i scrapped 2 or 3 guns from old magnetic crt's in the past for the same sort of idea if it ever happens ? but after some cold cathode experiments i think i would need to make a better vacuum or i would burn the heaters out or quicker ,need some glass heating fusing skills as well before i would ever try it .
Question that may come to mind as to me at the time was how to remove an electron gun with out the imploding problem ,the answer for magnetic Crt's is easy the anode connection can be drilled its just a thin metal wall no glass here so just let it then pressurize its a very subtle wait till the air filling the crt stops .
Well at the time it was these CRT's were going to the dump and what if came to mind .
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Thu Jul 07, 2016 10:27 pm

Harry, I doubt if the cathode would survive such an action. As far as I know they were activated after the tube was evacuated. The surface of cathodes was a metal that oxidizes very soon, so air in the tube is the definite end of the cathode.

The large tubes of the photo also had a deflection angle that was not so large, guess about 40 degrees.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Thu Jul 07, 2016 11:57 pm

Klaas Robers wrote:Harry, I doubt if the cathode would survive such an action. As far as I know they were activated after the tube was evacuated. The surface of cathodes was a metal that oxidizes very soon, so air in the tube is the definite end of the cathode.

The large tubes of the photo also had a deflection angle that was not so large, guess about 40 degrees.


Hi Klass
OH i didn't know that so that was a waste of time getting them then ...
i did enjoy trying the cold cathode experiments so stuck with those in any more High voltage near vacuum tests.

Tell you what those large crts in Steve's post look a bit of a worry stood up like that brave lady .

Edit...found this electrostatic tv by chance not big but first true b/w i have seen
http://members.iinet.net.au/~cool386/400tv/400tv.html
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Harry Dalek » Thu Jan 19, 2017 8:10 pm

Been near on 6 months since i have looked at this project some parts of it the CRT i have not really tested much till today with this simple test i have tried before on other Crts.
Working on the PMT camera has taken up a lot of time but no problems to have a little look at this again and do some more .
I power up the heater on 5 volts DC and the bug zapper inverter outputting 3000 volts less than half to the Anode and it was enough to get a cathode ray dot on the screen ....looks to me like the positive HV is ok should be fine on 7000 volts
Not overly bright to cause and problems just to show its operating .
Now something is a little confusing to me with tests with a UV light to the screen i have a green trace but electrons to screen it looks blue to me.
May be on full operating voltages it might be Green ? but here it is sure blue light by the looks of it on the phosphor screen.
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The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: 5AHP7A Vintage SSTV The Deep image

Postby Klaas Robers » Fri Jan 20, 2017 2:02 am

Harry, this is a P7 phosfor, which is a two layer phosfor. The most inner layer gives a blue light, you can see that if you look from the back side into the CRT. The front layer is a long decay yellow phosfor. It is illuminated by the blue light and then gives a yellow after glow of several seconds.

If you swipe with a magnet around the neck of the tube you can deflect the spot and you will see that it leaves a yellow trace oon the screen. Normally in SSTV practice you place a yellow colour filter (yellow plexiglass) in front of the screen, which will be untransparent for the blue illumination spot and you see only the yellow after glow. I placed then a circular polarized Polaroid filter in front of all. This gives a better contrast as it attenuates the surrounding light twice and the light from the screen only once. That makes the screen much darker.
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